The present invention relates generally to data transmission in communication systems and more specifically to methods and systems for facilitating multi-carrier operation in a mobile communication system.
As used herein, the terms “user equipment” and “UE” can refer to wireless devices such as mobile telephones, personal digital assistants (PDAs), handheld or laptop computers, and similar devices or other user agents (“UAs”) that have telecommunications capabilities. In some embodiments, a UE may refer to a mobile, wireless device. The term “UE” may also refer to devices that have similar capabilities but that are not generally transportable, such as desktop computers, set-top boxes, or network nodes.
In traditional wireless telecommunications systems, transmission equipment in a base station or other network node transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an evolved universal terrestrial radio access network (E-UTRAN) node B (eNB) rather than a base station or other systems and devices that are more highly evolved than the equivalent equipment in a traditional wireless telecommunications system. Such advanced or next generation equipment may be referred to herein as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment can be referred to as an evolved packet system (EPS). Additional improvements to LTE systems and equipment will eventually result in an LTE advanced (LTE-A) system. As used herein, the phrase “base station” will refer to any component, such as a traditional base station or an LTE or LTE-A base station (including eNBs), that can provide a UE with access to other components in a telecommunications system.
In mobile communication systems such as the E-UTRAN, a base station provides radio access to one or more UEs. The base station comprises a packet scheduler for dynamically scheduling downlink traffic data packet transmissions and allocating uplink traffic data packet transmission resources among all the UEs communicating with the base station. The functions of the scheduler include, among others, dividing the available air interface capacity between UEs, deciding the transport channel to be used for each UE's packet data transmissions, and monitoring packet allocation and system load. The scheduler dynamically allocates resources for Physical Downlink Shared Channel (PDSCH) and Physical Uplink Shared Channel (PUSCH) data transmissions, and sends scheduling information to the UAs through a control channel.
To facilitate communications, a plurality of different communication channels are established between a base station and a UE, among other channels, a Physical Downlink Control Channel (PDCCH). As the label implies, the PDCCH is a channel that allows the base station to send control signal to a UE for uplink and downlink data communications. To this end, the PDCCH is used to transmit scheduling assignment and control data packets referred to as Downlink Control Information (DCI) packets to the UE to indicate scheduling information to be used by the UE to receive downlink communication traffic packets on a Physical Downlink Shared Channel (PDSCH) or transmit uplink communication traffic packets on a Physical Uplink Shared Channel (PUSCH) or specific instructions to the UE (e.g., power control commands, an order to perform a random access procedure, or a semi-persistent scheduling activation or deactivation). A separate DCI packet may be transmitted by the base station to a UE for each traffic packet/sub-frame transmission.
In some cases, carrier aggregation can be used to support wider transmission bandwidths and increase the potential peak data rate for communications between a UE, base station and/or other network components. In carrier aggregation, multiple component carriers are aggregated and may be allocated in a sub-frame to a UE as shown in FIG. 1. FIG. 1 shows carrier aggregation in a communications network where each component carrier has a bandwidth of 20 MHz and the total system bandwidth is 100 MHz. As illustrated, the available bandwidth 100 is split into a plurality of carriers 102. UE 10 may receive or transmit on multiple component carriers (up to a total of five carriers 102 in the example shown in FIG. 1), depending on the UE's capabilities. In some cases, depending on the network deployment, carrier aggregation may occur with carriers 102 located in the same band and/or carriers 102 located in different bands. For example, one carrier 102 may be located at 2 GHz and a second aggregated carrier 102 may be located at 800 MHz.
In multi-carrier communications network implementations, various types of carriers can be defined. Backwards compatible carriers include carriers accessible to UEs that comply to a version or release of the specification prior to the version of release of the specification in which the support of carrier aggregation is added. In other words, backwards compatible carriers are accessible to UEs that are do not support and are not aware of carrier aggregation. Such UEs can be referred to as legacy UEs. For example, if carrier aggregation is added to LTE release 10, then backwards compatible carriers are accessible to UEs of earlier LTE releases such as LTE release 8 or LTE release 9. Backwards compatible carriers can be operated as a single carrier (stand-alone) or as a part of a carrier aggregation. In the case of frequency division duplexing (FDD) implementations, backwards compatible carriers may occur in pairs (e.g., DL (downlink) and UL (uplink) carrier pairs). Non-backwards compatible carriers are not accessible to UEs of earlier LTE releases, but are accessible to UEs of the LTE release that defines the operation of carrier aggregation. Non-backwards compatible carriers can be operated as a single carrier (stand-alone) if the non-backwards compatibility originates from the frequency duplex distance, or otherwise may be operated as a part of a carrier aggregation. An extension carrier cannot be operated as a single carrier (stand-alone), but must be a part of a component carrier set where at least one of the carriers in the set is a stand-alone-capable carrier. In multi-carrier networks, a UE DL Component Carrier Set includes the set of DL component carriers on which a UE may be scheduled to receive the PDSCH in the DL. Similarly, a UE UL Component Carrier Set includes the set of UL component carriers on which a UE may be scheduled to transmit the PUSCH in the UL.
Of the various carriers in a multi-carrier system, the carriers may generally be allocated into one of two types. Type A carriers are fully configured carriers that include all the sync channels and system information broadcasts necessary to allow all UEs to camp including legacy UEs and UEs that support or are aware of carrier aggregation. A Type A carrier is a backward compatible carrier if it supports legacy UEs. A Type A carrier is a non-backward compatible if it only supports UEs that support or aware of carrier aggregation. Type B carriers may not provide all the necessary system information broadcasts and may or may not include the sync channels. Type B carriers do not allow idle-mode UEs to camp. Similar to the extension carrier, Type B carriers may only serve RRC_CONNECTED UEs in carrier aggregation mode, i.e., a Type B carrier may not be a stand-alone carrier. Finally, Type B carriers may or may not include a PDCCH.
FIG. 2 is an illustration of an example network 50 that uses carrier aggregation. In FIG. 2, two base stations 52 and 54 (e.g., eNBs) communicate with several UEs. In this example, each of base stations 52 and 54 control 3 ‘cells’. In this illustration, the term cell may be used to refer to a certain geographical coverage area (although it should be noted that there may be small differences in coverage provided by the different carrier frequencies due to different propagation characteristics of the different frequencies). Cells A, B, C and D each operate using 3 different carrier frequencies 1, 2 and 3 and each carrier frequency further corresponding to a component carrier. Cell E operates using 2 different carrier frequencies and cell F operates using a single carrier frequency. The carrier frequencies used by each ‘cell’ depend on the deployment of the network and may be statically configured, or change infrequently. In the example, UEs 56 and 58 are both capable of operating using carrier aggregation. UE 58 is located within cell A and, as such, base station 52 may choose to use up to 3 carrier frequencies to communicate with UE 58. In contrast, UE 56 is located within cell F. Because cell F only provides a single carrier frequency, base station 54 communicates with UE 56 via a single carrier frequency only (e.g., carrier frequency 3).
FIG. 3 is an illustration of a multi-carrier network implementation and shows 4 component carriers (Frequencies 1-4) operated by the same base station (e.g., an eNB). As illustrated, the component carriers are not all adjacent in frequency and may even reside in different radio frequency bands. In this example, frequencies 1, 2 and 3 are Type A carriers, while frequency 4 is a Type B carrier. In this example, the base station has configured UE 60 to operate with frequency 3 as the UE's anchor carrier and frequency 4 as a non-anchor carrier of the UE. UE 62 is configured to operate with frequency 1 as the UE's anchor carrier and frequencies 2 and 3 as non-anchor carriers. During operation, the base station may reconfigure any of the UEs to change the anchor and non-anchor carriers upon which the UEs are operating (i.e., there may be a dynamic association between the UE and the carriers on which the UE is operating). In this example, UE 64 represents a UE that is not capable of operating in carrier aggregation mode. For example, UE 64 may be a UE that was built to an earlier version of the specification prior to the introduction of carrier aggregation. As such, UE 64 is configured to only operate using frequency 2.
In the example shown in FIG. 3, communication of user data and/or layer 3 control signaling (e.g., dedicated radio resource control (RRC) signaling) between the base station and UE 60 may use the anchor carrier (freq 3), the non anchor carrier (freq 4), or both. This behavior may be adjusted based upon the decisions of the scheduler within the base station.
Generally, in existing multi-carrier communications network implementations, although many different categories of component carriers (CCs) may be defined, the detailed operation of how a UE is assigned one or multiple of the CCs, the relationship across the multiple CCs and a UE, and the details of a downlink/uplink (DL/UL) CC set for a particular UE are not defined. Additional issues to be considered in -carrier aggregation implementations include whether a CC is qualified as a cell. Also, if a CC is qualified as a cell, the appropriate operation when a UE is assigned multiple CCs is undefined. Similarly, in multi-carrier implementations, existing standards fail to describe how the assignment and activation of a CC to a UE is performed, how a UE switches from one CC to another, how to define the CCs assigned to a particular UE, and how to scramble the data and control channels on each of the CCs assigned to the UE. Similarly, existing multi-carrier network implementations fail to provide mechanisms allowing a legacy UE to distinguish a non-backward compatible carrier from a backward compatible carrier.